This invention relates to railroad flat cars of the type used to transport semi-trailers in what is commonly referred to as piggyback service. One of the difficulties faced by railroads providing piggyback service is matching the railroad car fleet to the various sizes of trailers encountered. Economical operation requires maximum flexibility in car loading without wasting car space. Thus, it is desirable to arrange the car so that trailers of various sizes can be accommodated.
To illustrate the problem, in the past it has been common to carry two 45-foot trailers on one 90-foot car. Somewhat shorter trailers could also be accommodated, two on a car. However, trailers longer than 45 feet are now being utilized. For example, 57-foot trailers are now in service. Obviously, two 57-foot trailers cannot be loaded on a single 90-foot car. Loading one such trailer on a car leaves approximately one-third of the car's length unused. In effect, then, for every three 57-foot trailers loaded one-on-one on 90-foot cars, the space equivalent of one complete car is wasted.